The Study On The Sedimentology And Chronology Of Late Precambrian Strata In Nanzheng-Ningqiang Region

Nanzheng-Ningqiang region is located in the northern margin of the Yangtze plate, where late Precambrian strata is well distributed. On the basis of detailed field geological work and indoor research, the sedimentary facies markers are studied in detail including the petrology, texture, structure and sedimentary sequence of the late Precambrian strata, and sedimentary facies are distinguished. The zircon U-Pb geochronology of the samples from this strata is studied also. The following conclusions can be drawn from these study.1.The glacial deposit is distinguished from the Nanhua system of this region, and can be further identified as the periglacial littoral subfacies according to the lithology and sedimentary structure. The grain size of sediments of Nanhua system is fining-wpward, which reflected the sea water from shallow to deep, and also indicated the glacial deposits accompanied by transgression in the same time.2. The littoral subfacies is distinguished fron the Doushantuo Formation. Then the transgression of the Dengying Formation continued to expand, the algal dolomite member of the Dengying Formation was deposited at tidal flat; the Gaojiashan member was deposited at neritic tempesitites, in addition, the lower part of this member displayed the distal tempesitites, and the upper part displayed proximal tempesitites. Overall, the transgression continued to expand, the Beiwan member of the Dengying Formation is the carbonatite deposited in the tidal flat,which indicated transgression continued.3. The results from zircon U-Pb isotope dating suggested that the youngest age of571±21Ma in the Gangchang profile, which can represent the latest sedimentary age of Sinian system and also indicate the absence of Nanhua system in this profile. The results from the most of the samples concentrates between the age of700and800Ma which indicate that it is relative with the breakup of Rodinia.